resonant scattering at these depths (Table V). Although the specimens of 

 Cyclothone probably all had gas-filled swimbladders (Marshall, 1960, p. 66), 

 the bubble size would be too small for these fish to contribute to resonant 

 scattering at 5-6 kHz, 



Fish concentration in the middle sample (800-600 m) was about seven 

 times less than the deep sample. Although the larger specimens of Cyclothone 

 fall in the proper size range, as computed above, for the 10.5-18.5 kHz layer, 

 these fish also are in the size range where the gas gland and rete have degenerated 

 and the lumen of the swimbladder has become invested with fat (Marshall, op.cit.). 

 Thus, it is highly unlikely that Cyclothone is an important contributor to scattering 

 in this layer. 



Gold and Van Schuyler (op.cit.) reasoned that the layers they observed 

 at 950 and 610 m did not migrate since the peaks representing these layers on 

 the logarithmic reverberation trace did not shift during migration periods. 

 It is possible, however, that these layers or portions of them did migrate later, 

 after the peak on the reverberation trace was masked by an increase in rever- 

 beration due to shallower migrant scatterers. 



With regard to the six-foot Isaacs-Kidd Midwater Trawl collections, it 

 is interesting that samples 8-Tl-C and D contains as many mesopelagic fish as 

 it does, since, according to the echosounder record, the main scattering layer 

 descended past the sampling depth almost 30 minutes earlier. Most of the 

 specimens are medium sized myctophids similar to those in the two earlier 

 samples of that tow. With the exception of one specimen of Notolychnus 

 valdiviae, all of the individuals in this sample are too large according to 

 Equation (7), to contribute to 12 kHz scattering in the upper 100 meters of the 

 water column. However, though these individuals do not account for 12 kHz 

 scattering they may be important contributors to scattering at lower frequencies. 

 This situation in tows 8-Tl-C and D illustrates once again, that although echo- 

 sounder recordings generally provide an excellent graphic display of scattering 

 layers, they portray scattering only at frequencies near the operating frequency 

 of the instrument, usually 12 kHz. 



Besides the obvious correlation of scattering layer movements with changes 

 in light intensity, scattering layers have been found associated with an oxygen 

 minimum layer (< 0.08 ml O2/L.) in the Arabian Sea by Kinzer (1967). He also 

 reported the capture of myctophids and gonostomatids in large numbers from these 

 scattering layers. Kanwisher and Ebeling (op.cit.) found an abundance of 

 hatchetfish (sternoptychids) in the oxygen minimum (< 0.25 ml O2/L.) of the 

 eastern tropical Pacific. Many other authors including Marshall (1954, p. 176) 



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